Vibration is a known problem for human operators, users, passengers, etc. of equipment and vehicles. It is known to dampen vibration with passive, active, or semi-active damping techniques. These techniques have different merit for different problems, but generally, for high levels of vibration reduction, active control is required. This is essentially because passive damping generally lacks sufficient absorption efficiency, even over a narrow range of frequencies and moderate amplitudes. Passive damping also lacks adaptivity: a mass and spring of a given frequency, cannot adapt itself to an input force of changing frequency. Semi-active solutions are adaptive to changing frequencies, but generally reduce vibration less than desired, even if feedback and control are perfectly tuned.
Systems for vibration suppression of rotary-wing aircraft are specifically discussed in the literature. A variety of active or semi-active and passive systems are known. Helicopter vibration control has been examined at the source (rotor) with passive and active blade vibration control, as well as local solutions with passive damping and semi-active damping of seats. The vibration control of local structures such as the trim panels, seat structures and the seat cushions are tempting because these systems are subject to fewer certification requirements, offering easier implementation, as well as lower weight penalties [9] than rotor-local vibration suppression strategies.
Neck strain and back injuries are common health problems among, for example, the pilots and co-pilots of rotary-wing aircraft [1, 8]. Vibration from the blades through the fuselage to the human body has been found to create a wide range of health issues: from short term effects such as discomfort and fatigue to long term effects like chronic pain and spinal misalignment [1, 8].
All vibration frequencies are not equally harmful for humans, and it is far easier to design a system that improves a narrow range of vibration frequencies, than one that suppresses vibrations effectively across a wide spectrum. For instance, Hiemenz et al. [7] integrates two MagnetoRheological Fluid (MRF) dampers on the side columns of a SH-60 Seahawk crew seat. MRF dampers are “semi-active” vibration control systems, because they are given feedback to respond to current load conditions, but do not supply any force to counter the vibration in response, but rather change the damping properties of the dampers to increase the absorption of the materials given the current dynamics. MRF dampers rely on a material that is sensitive to magnetic field changes, and a field generator. While MRF dampers generally have lower energy consumption than active control strategies, they may be wanting in efficiency of damping.
Using experimental test results, Hiemenz et al. shows that the new system reduced the dominant rotor-induced vertical vibration (4/rev) by 76% for a 50th percentile male aviator. However, the 76% reduction in the 4/rev component does not result in a desired level of improvement on the pilot's comfort or well-being, mainly because the human body is less sensitive to the 4/rev component than lower harmonics, especially 1/rev. Although the transmissibility of the modified Seahawk seat is considerably reduced for medium and high frequency ranges (above 8 Hz) such as 4/rev component, it does not show significant improvements for the lower frequency range (between 0.5 Hz to 8 Hz) where the 1/rev excitation is expected. In some cases, the transmissibility of the modified seat is even higher than the unmodified seat in the lower frequency range. Furthermore, it should be noted that integrating the MRF dampers into the structure of the seat requires modification and certification of the existing seats for every type of helicopter.
As another example in the helicopter vibration context, consider various pilots with different weights using a same seat cushion system. Chen et al. [2, 3] evaluate the performance of different seat cushion materials for mitigating whole body vibration (WBV) exposure in a Bell-412 helicopter. It is demonstrated that the use of a meticulously designed cushion reduces the vibration level by 24.9% in terms of health risk for a 50th percentile pilot, but it only reduces vibration level by a value of 0.2% for a 85th percentile co-pilot. While this study was limited to a passive system, it illustrates the difficulties inherent in the problem.
Active control feedback systems have known advantages in terms of damping efficiency, and rely on sensors, feedback electronics and a power supply. The main function of active systems is to add energy to the system by applying a time varying force, with a same magnitude as an observed force, but opposite in phase. The applied forces are usually generated using such active elements as pneumatic, hydraulic, piezo-electric or electromechanical actuators, and they require a relatively large amount of power to operate compared with passive or semi-active devices. While the design of active control systems is research-intensive and costly, it demonstrates a better performance than passive or semi-active control systems.
Chen et al. [4-6] integrated two types of actuators into the helicopter seat structure to dampen unwanted vibration: an electromechanical motor and a piezo-electric actuator. In both cases, significant reduction in vibration level is achieved, for instance, in the latter case, it is reported that overall 26% vibration reduction at the pilot helmet location is achieved. Despite the fact that the active seat performance showed improvement in the overall vibration, major areas of improvement were identified:                The active control results did not show any appreciable reduction to the low frequency vibration, namely, 1/rev harmonic.        The active seat design does not comply with the crashworthiness requirements of helicopter seats.        The piezo-electric actuator lacked a displacement suitable to cancel vibration amplitudes observed.        The bulky size of the electromechanical motor was an undesirable aspect of this type of actuator, and is better suited to lab testing than deployment in aircraft.        
US2013/0180350 to Kraus et al. teaches an active bearing for vibration reduction. The active bearing preferably includes a support element for supporting a static load transfer. The support element “typically comprises a suspension spring element or a plurality of suspension spring elements” that is “preferably produced from a material with small damping capacity so that the best possible insulation effect is enabled between vibrating load 8 and support unit 6 at high excitation frequencies”. In the embodiment of FIG. 4, the support element is an elastomer molded body that acts as a spring. It is understood that some elastomeric materials behave as springs in their response, while other elastomers are much more like dampers, and visco-elastic materials are somewhere between these extremes. The teachings here suggest the substitution of a suspension spring element with an equivalent elastomeric spring. A dashpot or damper is used to decouple the support element 3 from the dynamic load bearing (force path II). The encasing of an active element for counteracting a dynamic load, with a passive element for transmitting a static load, is shown in FIG. 4.
US 2013/0328253 (253) is addressed not to a support for specifically suppressing 0.5-8 Hz vibrations, but to a system for suppressing all vibrations that interfere with atomic force microscopes, electron microscopes, etc. The teachings include an intermediate mass that is isolated in 6 DoF, and has a plurality of voice coil motors, passive dampers, and springs along different axes to isolate an intermediate mass from the sprung mass and the floor, in order to improve vibration reduction at the sensitive equipment. “Voice coil motors” include a very wide range of devices, from devices in headphones to devices that simulate vibrations of rockets. Given that '253 is directed to avoiding the “small payload vibrations” to which such instruments are “very sensitive” (clearly nobody would put such sensitive equipment in a high amplitude vibration environment that would be of any concern to a human occupant), and given that a “small and inexpensive actuator” is preferred, it is submitted that a small voice coil motor is inherent to this application. At [0006] the patent does not indicate that the extremely small displacement of piezoelectric actuators is problematic, but that voice coil motors can be used unlike the suggestion in U.S. Pat. No. 5,660,255 to reduce cost. Small and inexpensive voice coil motors are beneficially compact, but are not powerful enough to effectively drive higher amplitude vibrations encountered in many applications, even if such voice coil motors are likely sufficient to cancel noise in a generally quiet lab for operating such scientific equipment. The addition of an intermediate mass, with clearances required for its movement, and the multiplication of joints, makes compactness a problem, even if the design could be modified to provide the force and actuation length required for application in noisier environments.
Patent applications, such as US 2014/0263932 and US 2013/0092814 show active vibration cancellation systems with a spring and an actuator with suitable linkages that appear to be compact and suitable for deployment under a seat. Suitability for suppressing 0.5-8 Hz vibrations is not considered or discussed, and is not inferable.
In rotorcraft seats, as in many stations for operating, monitoring, or supporting people near, vibrating equipment, there is a need for a compact, lightweight, active system for cancelling vibrations.